Semiconductor device and method for manufacturing the same

ABSTRACT

A trench-gate type transistor has a gate insulating film formed on an inner wall of a trench. The gate insulating film includes a first portion located on a wall of the trench and a second portion located on upper and bottom portions of the trench. The first portion includes a first oxide film, a nitride film, and a second oxide film. The second portion includes only an oxide film and is thicker than the first portion. Accordingly, electric field concentration on upper and lower corner portions of the trench can be reduced to improve the withstand voltage. In addition, and end of the trench may have an insulation layer that is thicker than the first portion.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patent application Ser. No. 09/758,377, which was filed on Jan. 12, 2001.

[0002] The contents of the parent application, U.S. patent application Ser. No. 09/758,377, are incorporated herein by reference. Also, this application is based upon and claims the benefit of Japanese Patent Applications No.2001-187127, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0003] This invention relates to a semiconductor device, in which a trench is formed in a semiconductor substrate with a layered film formed on an inner wall of the trench, and to a method for manufacturing the same.

[0004] U.S. Pat. No. 5,321,289 (which corresponds to Japanese unexamined patent publication JP-A-6-132539) discloses such a transistor, as shown in FIG. 20, having a trench-gate structure in which a trench is formed on a semiconductor substrate and a gate insulating film composed of an oxide film and a nitride film is formed on an inner wall of the trench. Because the gate insulating film is composed of a compound film of the oxide film and the nitride film, the device can provide a higher gate withstand voltage and a lower on-voltage than a device in which the gate insulating film is composed of only an oxide film.

[0005] As a result of studies of the semiconductor device described above, however, it was found that electric field tends to concentrate on corner portions of the upper and bottom portions of the trench, which lowers the withstand voltage. Further, the gate insulating film composed of the oxide film and the nitride film has many interface states. In this connection, it was further found that a threshold voltage was liable to vary due to effects of the interface states at a transistor operation state. This can lower the reliability of the device.

SUMMARY OF THE INVENTION

[0006] The present invention has been made in view of the above problems. An object of the present invention is to attain a high withstand voltage in a semiconductor device having a trench-gate structure and simultaneously to prevent a decrease in withstand voltage by relaxing electric field concentration at upper and bottom portions of a trench. Another object of the present invention is to suppress variations in threshold voltage while keeping the high withstand voltage in the semiconductor device.

[0007] According to the present invention, briefly, an insulating film disposed on an inner wall of a trench has a first portion and a second portion. The first portion is composed of a first oxide film, a nitride film, and a second oxide film, and the second portion is composed of only an oxide film. Further, one of the first portion and the second portion of the insulating film is disposed on a side wall of the trench, and another one of the first portion and the second portion of the insulating film is disposed on at least one of an upper portion and a bottom portion of the trench.

[0008] Specifically, when the first portion is disposed on the side wall portion of the trench, the second portion is disposed on at least one of the upper portion and the bottom portion of the trench. Accordingly, electric filed concentration on the one of the upper portion and the bottom portion can be mitigated, and a high withstand voltage can be attained.

[0009] The first portion may be disposed only on the bottom portion of the trench. In this case, the second portion is disposed on the side wall portion of the trench. Accordingly, variations in threshold voltage can be suppressed while keeping a high withstand voltage.

[0010] In another aspect, in a semiconductor device of the present invention, a gate-insulating film is thicker at a longitudinal end of a trench, which is formed in a surface of a semiconductor substrate, than the rest of the trench to provide higher breakdown voltage without increasing ON voltage. A heavily doped region, which is heavily doped with an impurity, is formed in a trench surface, which defines the trench, at the longitudinal end. A silicon oxide film, which is formed on the trench surface, is thicker at the longitudinal end than the rest of the silicon oxide film because the heavily doped region is oxidized faster. Therefore, the gate-insulating film is thicker at the longitudinal end. Alternatively, the trench surface is oxidized after removing the silicon nitride film, which reduces oxidization speed of silicon, at the longitudinal end. Thus, the silicon oxide film is thicker at the longitudinal end.

[0011] A semiconductor device in the present invention includes a source region, which is located in the surface of the semiconductor substrate and includes a first region and a second region. The first region has a predetermined impurity concentration and a predetermined depth from the surface of the substrate. The second region has an impurity concentration lower than the first region and a depth larger than the first region. The second region is located at an entrance of the trench to separate the first region and a channel region, so a thickness of the gate-insulating film on the channel region is not affected by a thick silicon oxide film formed on the first region. Thus, the semiconductor device in the present invention has stable threshold voltage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiments described below with reference to the following drawings, in which;

[0013]FIG. 1 is a cross-sectional view showing a semiconductor device in a first preferred embodiment of the invention;

[0014]FIGS. 2A to 2H are cross-sectional views showing steps of a method for manufacturing the semiconductor device shown in FIG. 1;

[0015]FIGS. 3A and 3B are cross-sectional views schematically showing states of upper and bottom portions of a trench, respectively, formed by the method shown in FIGS. 2A to 2H;

[0016]FIGS. 3C and 3D are cross-sectional views showing states of upper and bottom portions of a trench, respectively, formed by a conventional method, for comparison;

[0017]FIG. 4 is a schematic diagram illustrating silicon residues (black-Si) that can be produced at the bottom portion of the trench;

[0018]FIGS. 5A to 5C are schematic diagrams illustrating suppression of the effects of silicon residues by the method shown in FIGS. 2A to 2H;

[0019]FIG. 6 is a cross-sectional view showing a semiconductor device, which is a modification of the embodiment of FIG. 1;

[0020]FIG. 7 is a cross-sectional view showing a semiconductor device, which is another modification of the embodiment of FIG. 1;

[0021]FIG. 8 is a cross-sectional view showing a semiconductor device in a second preferred embodiment of the invention; and

[0022]FIGS. 9A to 9H, and 10A to 10D are cross-sectional views showing steps of a method for manufacturing the semiconductor device shown in FIG. 8;

[0023]FIG. 11A is a cross-sectional view of a semiconductor device according to the third embodiment of the present invention;

[0024]FIG. 11B is a partial side cross-sectional view of a third embodiment seen from a direction that is perpendicular to the view direction of FIG. 11A;

[0025]FIGS. 12A to 12C are cross-sectional views showing first steps of a process for manufacturing the semiconductor device according to the third embodiment;

[0026]FIGS. 12D to 12F are partial side cross-sectional views showing first steps seen from directions perpendicular to the view directions of the corresponding views of FIGS. 12A to 12C, respectively;

[0027]FIGS. 13A to 13C are cross-sectional views showing intermediate steps of the process following FIG. 12C;

[0028]FIGS. 13D to 13F are partial side cross-sectional views seen from directions perpendicular to the view directions of the corresponding FIGS. 13A to 13C, respectively, to show the intermediate steps following FIG. 12F;

[0029]FIGS. 14A and 14B are cross-sectional views showing final steps of the process following FIG. 13C;

[0030]FIGS. 14C and 14D are partial cross-sectional views of the final steps following FIG. 13F seen from directions perpendicular to the view directions of corresponding FIGS. 14A and 14B, respectively;

[0031]FIG. 15A is a cross-sectional view of a semiconductor device according to the fourth embodiment of the present invention;

[0032]FIG. 15B is a partial cross-sectional view of a fourth embodiment seen from a direction perpendicular to the view direction of FIG. 15A;

[0033]FIGS. 16A to 16C are cross-sectional views showing first steps of a process for manufacturing the semiconductor device according to the fourth embodiment;

[0034]FIGS. 16D to 16F are partial side cross-sectional views showing early steps as seen from view directions perpendicular to the view directions of corresponding FIGS. 16A to 16C, respectively;

[0035]FIGS. 17A to 17C are cross-sectional views showing intermediate steps of the process following FIG. 16C;

[0036]FIGS. 17D to 17F are side cross-sectional views of intermediate steps following FIG. 16F as seen from directions perpendicular to the view directions of corresponding FIGS. 17A to 17C, respectively;

[0037]FIGS. 18A and 18B are cross-sectional views showing final steps of the process following FIG. 17C;

[0038]FIGS. 18C and 18D are partial side cross-sectional views showing final steps following FIG. 17F as seen from directions perpendicular to the view directions of FIGS. 18A and 18B, respectively;

[0039]FIG. 19 is a cross-sectional view of a semiconductor device according to the fifth embodiment of the present invention;

[0040]FIG. 20 is a cross-sectional view of a related semiconductor device; and

[0041]FIG. 21 is a fragmentary plan view showing a longitudinal end of a trench.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] First Embodiment

[0043] A semiconductor device according to a first preferred embodiment is explained with reference to FIG. 1, which has a transistor such as a power MOSFET, an IGBT, or the like having a trench-gate structure.

[0044] In FIG. 1, an n-type drift layer 2 is formed on a p⁺ or n⁺-type silicon substrate 1, and a p-type layer 3 is formed thereon as a base region. An n⁺-type layer 4 is formed in the p-type layer 3 to form a source region. A semiconductor substrate 5 is composed of these parts. On a main surface of the semiconductor substrate 5, a trench 6 is formed and penetrates the n⁺-type layer 4 and the p-type layer 3 to reach the drift layer 2. A gate insulating film is formed on an inner wall of the trench 6.

[0045] The gate insulating film is a layered film that is formed on the side wall portion of the trench 6 and a silicon oxide film 7 d formed on the upper and lower portions of the trench 6. The layered film is composed of a silicon oxide film (first oxide film) 7 a, a silicon nitride film 7 b, and a silicon oxide film (second oxide film) 7 c. The silicon nitride film 7 b is positioned with an upper edge that is positioned at an upper portion than the boundary between the p-type layer 3 and the n⁺-type layer 4, i.e., on the main surface side of the semiconductor substrate 5. The silicon oxide films 7 d, 7 e formed on the upper portion and the bottom portion of the trench 6 respectively have thicknesses thicker than that of the layered film formed on the side wall portion of the trench 6. Here, the upper portion of the trench 6 is that part including the upper side corner portion of the trench 6, while the bottom portion of the trench 6 is that part including the bottom side corner portion of the trench 6.

[0046] In the trench 6, a gate electrode 8 is formed from doped polycrystalline silicon. A BPSG film 9 is formed on the surfaces (substrate main surface) of the p-type layer 3 as the base region and the n⁺-type layer 4 as the source region, and a source electrode 10 and a metallic film for gate and collector electrodes (not shown in FIG. 1) are formed to be connected through contact holes formed in the BPSG film 9.

[0047] The constitution described above can provide a transistor having a gate-trench structure in which the insulating films formed on the inner wall of the trench 6 collectively form a gate insulating film, and the region of the side wall portion of the trench 6 in the p-type layer 3 functions as a channel region.

[0048] Here, as the gate insulating film, the layered film composed of the silicon oxide film 7 a, the silicon nitride film 7 b, and the silicon oxide film 7 c is formed on the side wall portion of the trench 6. This structure can provide a high withstand voltage as in a conventional one. In addition, because the silicon oxide films 7 d, 7 e formed on the upper and bottom portions of the trench 6 have thicknesses thicker than that of the layered film on the side wall portion of the trench 6, electric field concentration is mitigated on the upper and lower corner portions of the trench 6, thereby preventing the decrease in withstand voltage at that portions.

[0049] Next, a method for manufacturing the semiconductor device described above is explained with reference to FIGS. 2A to 2H.

[0050] First, in a step shown in FIG. 2A, the n⁻-type drift layer 2 is formed on the P⁺-type or n⁺-type silicon substrate 1. Then, the p-type layer 3 and the n⁺-type layer 4 as the source region are sequentially formed by ion-implantation and thermal diffusion. The depth of the p-type layer 3 is about 2 to 3 μm, and the depth of the n⁺-type layer 4 is about 0.5 μm.

[0051] In a step shown in FIG. 2B, a silicon oxide film 11 is deposited by a CVD method as a trench mask with a thickness of about 0.5 μm, and patterning is performed thereto by photo-lithography and anisotropic dry etching. Then, the trench 6 is formed to penetrate the n⁺-type layer 4 and the p-type layer 3 and to reach the drift layer 2 by anisotropic dry etching using the patterned silicon oxide film 11 as a mask. The depth of the trench 6 is about 4 to 6 μm.

[0052] Subsequently, in a step shown in FIG. 2C, silicon exposed in the trench 6 is isotropically etched and removed at a depth of about 0.1 μm by chemical dry etching using CF₄ and O₂ gases. Then, a sacrifice oxide film of about 100 nm is formed by thermal oxidation in H₂O or O₂ atmosphere. After that, the sacrifice oxide film is removed by wet etching using dilute hydrofluoric acid. At that time, the oxide film 11 as the trench mask is simultaneously etched.

[0053] The time period for the wet etching can be set at either of a time period for removing only the sacrifice oxide film and a time period for removing both the sacrifice oxide film and the silicon oxide film 11. After that, the silicon oxide film 7 a is formed to have a thickness of about 100 nm by thermal oxidation performed in H₂O or O₂ atmosphere.

[0054] Next, in a step shown in FIG. 2D, the silicon nitride film 7 b is formed to have a thickness of about 10 to 30 nm by an LPCVD method.

[0055] In a step shown in FIG. 2E, the part of the silicon nitride film 7 b disposed on the bottom portion of the trench 6 is removed by anisotropic dry etching using gas including CHF₃ and O₂ while remaining the silicon nitride film 7 b on the side wall portion of the trench 6, so that the silicon oxide film 7 a is exposed. At that time, that parts of the silicon nitride film 7 b formed on the upper portion of the trench 6 and on the silicon oxide film 11 on the substrate surface are removed simultaneously, and the silicon oxide film 7 a is exposed at that regions.

[0056] In a step shown in FIG. 2F, for example, thermal oxidation is carried out in H₂O or O₂ atmosphere at preferably 850 to 1050° C., and accordingly, the silicon oxide film 7 c of about 5 to 10 nm is formed on the silicon nitride film 7 b. At that time, the silicon oxide films 7 d, 7 e are formed on the upper and lower portions of the trench 6, where the silicon nitride film is removed, to have a thickness of about 180 to 330 nm respectively that is increased due to thermal oxidation.

[0057] In this case, because the silicon nitride film 7 b is thin with a thickness of about 10 to 30 nm, the silicon oxide film can be grown on the upper portion of the trench 6 in a lateral direction after the silicon nitride film 7 b is partially etched. Therefore, the thickness of the silicon oxide film can be increased not only on the surface of the silicon substrate but also at the opening portion of the trench 6. That is, the thickness of the silicon oxide film can be increased as well at the corner portion of the trench 6.

[0058] Subsequently, in a step shown in FIG. 2G, doped polycrystalline silicon 8 for the gate electrode 8 is formed by the LPCVD method, thereby filling inside the trench 6. Successively, the polycrystalline silicon 8 is etched-back to have a desired thickness. In a step shown in FIG. 2H, then, the polycrystalline silicon 8 is patterned by photo-lithography to form the gate electrode 8.

[0059] After that, as shown in FIG. 1, the BPSG film 9, as an intermediate insulating film, is formed by a plasma enhanced CVD method. The contact holes are formed in the BPSG film 9 by photo-lithography and anisotropic dry etching, and the metallic films for the source, gate and collector electrodes are formed by a sputtering method.

[0060] Thus, the semiconductor device shown in FIG. 1 is manufactured. In the method described above, an IGBT is exemplified as the semiconductor device; however, dimensions of the respective parts can be changed appropriately. For example, when a power MOSFET (DMOS) is manufactured as the semiconductor device, preferably, the depth of the trench 6 is about 2 μm, the depth of the p-type layer (p well) 3 is about 1.5 μm, the depth of the n⁺-type layer 4 is about 0.5 μm, and the thickness of the silicon oxide film 7 a is about 50 nm.

[0061] According to the manufacturing method described above, after the silicon oxide film 7 a and the silicon nitride film 7 b are formed on the inner wall of the trench 6, the silicon nitride film 7 b on the upper and bottom portions of the trench 6 is removed and then thermal oxidation is performed. By performing this thermal oxidation, the silicon oxide film 7 c is formed on the silicon nitride film 7 b and simultaneously, the silicon oxide films 7 d, 7 e having large thicknesses are formed on the upper and bottom portions of the trench 6 where the silicon nitride film are removed.

[0062]FIGS. 3A and 3B respectively show portions of the gate insulating film on the upper and lower portions of the trench manufactured by the method described above, and illustrations of FIGS. 3A and 3B correspond to practical cross-sectional photographs. Also FIGS. 3C and 3D show portions of the gate insulating film on the upper and lower portions of the trench manufactured by a conventional method in which no removal of the silicon nitride film is performed, and illustrations of FIGS. 3C and 3D correspond to practical cross-sectional photographs. Incidentally, the difference between the examples shown in FIGS. 3A, 3B and FIGS. 3C and 3D is only whether the removal of the silicon nitride film is performed, and the other manufacturing conditions are substantially identical.

[0063] When the gate insulating film is formed by the conventional method, the upper and lower portions of the trench have a layered film as the gate insulating film. As shown in FIG. 3C, the thickness on the upper portion of the trench is 140 nm, while as shown in FIG. 3D, the thickness on the bottom portion is 70 nm.

[0064] To the contrary, when the gate insulating film is formed by the method according to the present embodiment described above, only the silicon oxide film exists on the upper and bottom portions of the trench. As shown in FIG. 3A, the thickness on the upper portion of the trench is 330 nm while, as shown in FIG. 3B, the thickness on the bottom portion of the trench is 180 nm. Therefore, when the silicon nitride film on the upper and bottom portions of the trench is removed and the thermal oxidation is carried out as in the present embodiment, the electric field concentration at the corner portions of the upper and bottom portions of the trench can be reduced, and a decrease in the withstand voltage at those locations can be prevented. Further, because the thickness of the silicon oxide film is thick at the upper and bottom portions of the trench 6, the gate input capacity can be reduced.

[0065] Incidentally, when the trench is formed by trench etching, as shown in FIG. 4, there is a case where silicon residues (black silicon) 6 a are produced during the formation of the trench 6. When the gate insulating film is formed at the region having such columnar silicon residues 6 a, electric field can locally concentrate on that portion to decrease the gate withstand voltage. Especially in power semiconductor elements such as a power MOS and an IGBT, since the gate region has a large area in a range of several dozens of square millimeters to several hundreds of square millimeters, the probability of adverse effects of the silicon residues is large.

[0066] In contrast, according to the manufacturing method described above, the adverse effects of the silicon residues can be eliminated. Specifically, in the case where columnar silicon residues 6 a are produced on the bottom portions of the trench 6, if the silicon oxide film is formed as in the conventional way, the effects of the silicon residues appear. However, in the present embodiment, even if silicon residues 6 a are produced as shown in FIG. 5A, since the portions of the silicon nitride film 7 b extending on the upper and bottom portions of the trench 6 are removed in the step shown in FIG. 2E, the state shown in FIG. 5B is provided.

[0067] Further, because the thermal oxidation is performed in the step shown in FIG. 2F, the thick silicon oxide film 7 e is formed on the bottom portion of the trench 6 to cover the entire region having the silicon residues 6 a, and the state as shown in FIG. 5C is provided. Accordingly, the decrease in gate withstand voltage is suppressed at the bottom portion of the trench 6 and a high gate voltage yield can be attained.

[0068] Incidentally, in the embodiment described above, the gate withstand voltage is improved by forming the insulating film on both the upper and bottom portions of the trench 6 only from the silicon oxide film; however, as shown in FIGS. 6 and 7, this countermeasure may be applied to only one of the upper and bottom portions of the trench 6, and the gate withstand voltage at another one thereof may be increased by another countermeasure. To form the single silicon oxide film at only one of the upper and bottom portions of the trench 6, for example, the portion of the silicon nitride film on another one of the upper and bottom portions is masked not to be removed during the dry-etching.

[0069] In the embodiment described above, the transistor having a trench-gate structure is exemplified as a semiconductor device; however, the withstand voltage can be increased even in other semiconductor devices such as a semiconductor device having a trench-type capacitor and a semiconductor device having an element isolation structure, by forming the insulating film on the inner wall of the trench, from the layered film of the oxide film and the nitride film on the side wall portion, and only from the oxide film on the upper portion and/or the -bottom portion of the trench. The conductive type of each layer in the semiconductor device is not limited to that shown in FIG. 1, and may be inverted from that shown.

[0070] Second Embodiment

[0071] A semiconductor device according to a second preferred embodiment is explained with reference to FIGS. 8 to 10, in which the structure of a gate insulating film is different from that in the first embodiment, and the differences are explained below. The same parts as those in the first embodiment are indicated by the same reference numerals.

[0072] The gate insulating film in the second embodiment is composed of a layered film formed on the bottom portion of the trench 6 and a silicon oxide film 107 d formed on the side wall portion and the upper portion of the trench 6. The layered film is composed of a silicon oxide film 107 a, a silicon nitride film 107 b and a silicon oxide film 107 c. The silicon nitride film 107 b has an upper end that is provided at a position lower than that of the boundary between the p-type layer 3 and the drift layer 2, i.e. at a back surface side of the semiconductor substrate 5.

[0073] Thus, in this embodiment, because the layered film composed of the silicon oxide film 107 a, the silicon nitride film 107 b, and the silicon oxide film 107 c is formed on the bottom portion of the trench 6, a high withstand voltage can be attained as in a conventional one. Further, because the oxide film 7 d formed on the side wall portion of the trench 6 is composed of only a silicon oxide film, variation in threshold voltage can be suppressed. As a result, the variation in threshold voltage can be decreased while maintaining a high gate withstand voltage.

[0074] Next, a manufacturing method of the semiconductor device described above is explained with reference to FIGS. 9A to 9H and 10A to 10D. Here, the steps shown in FIGS. 9A to 9D are substantially the same as those shown in FIGS. 2A to 2D, and therefore, the explanation is started from a step shown in FIG. 9E. Incidentally, the silicon oxide film 7 a in FIG. 2C corresponds to the silicon oxide film 107 a in FIG. 9C, and the silicon nitride film 7 b in FIG. 2D corresponds to the silicon nitride film 107 b in FIG. 9D.

[0075] Then, in the second embodiment, in the step shown in FIG. 9E, photo-resist 12 is embedded inside the trench 6 by a rotation coating method. It should be noted that the trench 6 may be filled with a material other than the photo-resist, such as poly silicon, provided that the material can serve as an etching stopper with respect to the silicon nitride film.

[0076] In a step shown in FIG. 9F, the photo-resist 12 is partially removed by anisotropic etch-back that is performed under conditions involving a selective ratio between the photo-resist and the silicon nitride film, and accordingly, the photo-resist 12 remains only on the bottom portion of the trench 6.

[0077] In a step shown in FIG. 9G, the silicon nitride film 107 b other than the portion covered with the photo-resist 12 at the bottom portion of the trench 6, i.e., the silicon nitride film 107 b disposed on the side wall portion of the trench 6 is removed by dry etching using gas including CHF₃ and O₂. At that time, the silicon nitride formed disposed on the upper portion of the trench 6 and on the silicon oxide film 11 on the substrate surface is removed simultaneously. Then, in a step shown in FIG. 9H, the photo-resist remaining at the bottom portion of the trench 6 is removed.

[0078] Next, in a step shown in FIG. 10A, for example, thermal oxidation is performed in H₂O or O₂ atmosphere at 950° C. The thermal oxidation forms the silicon oxide film 107 d that thickens on the side wall portion and the upper portion of the trench 6 with a thickness of about 100 nm. At the bottom portion of the trench 6, the silicon oxide film 107 c of several nanometers is formed on the silicon nitride film 107 b.

[0079] In a step shown in FIG. 10B, doped polycrystalline silicon 8 is formed by an LPCVD method for the gate electrode, thereby filling the trench 6. Successively, the polycrystalline silicon 8 is etched-back to have a desired thickness. In a step shown in FIG. 10C, the polycrystalline silicon 8 is patterned by photo-lithography, thereby forming the gate electrode 8.

[0080] Then, in a step shown in FIG. 10D, the BPSG film 9 is formed as an intermediate insulating film by a plasma enhanced CVD method, the contact holes are formed in the BPSG film 9 by photo-lithography and anisotropic dry etching, and metallic films for the source, gate, and collector electrodes are formed by a sputtering method. Thus, the semiconductor device shown in FIG. 8 is manufactured.

[0081] According to the manufacturing method described above, after the silicon oxide film 107 a and the silicon nitride film 107 b are formed on the inner wall of the trench 6, the portions of the silicon nitride film disposed on the side wall portion and the upper portion of the trench 6 are removed. After that, thermal oxidation is performed to form the silicon oxide film 107 c on the silicon nitride film 107 b, and to form the silicon oxide film 107 d on the side wall portion and the upper portion of the trench 6 where silicon nitride film is removed.

[0082] Therefore, the layered film composed of the silicon oxide film 107 a, the silicon nitride film 107 b, and the silicon oxide film 107 c is formed on the bottom portion of the trench 6, and accordingly, a high gate withstand voltage can be attained. Also, because only the silicon oxide film 107 d is formed on the side wall portion and the upper portion of the trench 6, variation in threshold voltage can be reduced. Also, the thickness of the silicon oxide film on the upper portion of the trench 6 can be increased by oxidation accelerated by the n⁺-type layer 4. Because of this, the electric field concentration on the corner portion of the upper portion of the trench 6 can be reduced, and a reduction in the withstand voltage at that portion can be suppressed. Incidentally, the conductive type of each layer in the semiconductor device is not limited to that shown in FIG. 8 and may be inverted from that shown.

[0083] Third Embodiment

[0084] In a transistor like that of the first embodiment, the stack of films may be thinner at an end of the trench 6, as shown in FIG. 21, in comparison to the rest of the stack because the end of the trench 6 is rounded. Therefore, improvement in the resistance of the gate-insulating film 7 against electric-field intensity may be limited by the thinner stack located at the end of the trench 6.

[0085] In addition, the entrance oxide film 7 d (See FIG. 1) is formed by oxidizing a source region 4. The source region 4 is oxidized faster because the source region 4 is heavily doped by an impurity. Although growing speeds of the first and second silicon oxide films 7 a, 7 c are suppressed by the silicon nitride film 7 b, the stack is thickened by the entrance oxide film 7 d at the top end.

[0086] If the top end overlaps with a channel region, which is generated in a surface of a p-type base region 3 in the trench 6, threshold voltage of the proposed device fluctuates.

[0087] The thickening of the stack is improved by thinning the silicon oxide films 7 d and 7 e. However, a nominal thickness of the stack is also reduced, so the durability of the gate-insulating film 7 is lowered. As an alternative, the source region 4 may be thickened enough to avoid having the top end overlap with the channel region.

[0088] However, the thermal diffusion distance needs to be increased to make the source region 4 thicker, so the size of the proposed device is enlarged, and the ON voltage of the proposed device is increased. The third embodiment is an attempt to solve these potential problems.

[0089] As shown in FIGS. 11A and 11B, a semiconductor device according to the third embodiment has a trench-gate structure, which is applied to a power MOSFET (Metal-Oxide-Silicon Field Effect Transistor) or an IGBT (Insulated Gate Bipolar Transistor). In FIGS. 11A and 11B, an n⁻-type (first conduction type) drift layer 22 is located on an n⁺-type or p⁺-type silicon substrate 21. On a surface of the n⁺-type drift layer 22, a p-type (second conduction type) base region 23 is located. Near the surface of the p-type base region 23, an n⁺-type (first conduction type) source region 24 is located. The silicon substrate 21, the drift layer 22, the base region 23, and the source region 24 form a semiconductor substrate. The semiconductor substrate has a trench 25, which extends vertically as viewed in FIG. 11A from a surface of the semiconductor substrate through and the base region 23 and reaches the drift layer 22. The source region 24 is located at the entrance of the trench 25. A channel region is generated in the base region 23 in the vicinity of the trench 25. As shown in FIG. 11B, a heavily doped region 26, which is heavily doped with an impurity, is located in a surface that defines the trench 25, at an end of the trench 25. As shown in FIG. 11B, the heavily doped region 26 extends from the entrance to the bottom of the trench 25, at the end of the trench.

[0090] A gate-insulating film 27 is located on the surface defining the trench 25. The gate-insulating film 27 includes entrance and bottom silicon oxide films 27 d and 27 e, which are located respectively at the entrance and the bottom of the trench 25, and a stack of films formed between the entrance and the bottom. The stack includes a first silicon oxide film 27 a, a silicon nitride film 27 b, and a second silicon oxide film 27 c. An upper end of the film 27 b in the vertical direction of FIG. 1A is located on a surface of the source region 24 A lower end of the film 27 b is located on a surface of the drift layer 22. The first and second silicon oxide films 27 d, 27 e are thicker than the stack of films. As shown in FIG. 11B, the first silicon oxide film 27 a is thicker on a surface of the heavily doped region 26 than the rest of the first silicon oxide film 27 a. Namely, the stack is thicker at the longitudinal end of the trench 25 than the rest of the stack.

[0091] A gate electrode 28 made of doped poly crystalline silicon is located on the gate-insulating film 27 in the trench 25. The gate electrode 28, the base region 23, and the source region 24 are covered by an interlayer insulating film 29, which is made of BPSG (boron phosphorus silicate glass). Through a contact hole made in the interlayer insulating film 29, a source electrode 210 is electrically connected to the base region 23 and the source region 24, and other electrodes (not illustrated) are respectively connected to the gate electrode 28 and a drain.

[0092] In the device in FIGS. 11A and 11B, the stack of films is thicker at the longitudinal end than at the rest of the stack. Therefore, the resistance of the gate-insulating film 27 against electric-field intensity is improved in comparison with the device of FIG. 1, so the breakdown voltage of the device of FIGS. 11A and 11B is increased without increasing the ON voltage.

[0093] The device in FIGS. 11A and 11B is manufactured through a process including steps shown in FIGS. 12A to 14D. As shown in FIGS. 12A and 12D, the n⁻-type drift layer 22 is formed on the n⁺-type or p⁺-type silicon substrate 21. Then, the p-type base region 23, the thickness of which ranges between 2 and 3 micrometers, is formed in the surface of the n⁻-type drift layer 22 using ion implantation and thermal diffusion. The n⁺-type source region 24, which has a thickness of 0.5 micrometers, is formed in the surface of the p-type base region 23. A silicon oxide film 211, which is deposited on the substrate 21, is defined by photolithography to form an opening in the silicon oxide film 211. Then, the trench 25, which has a depth ranging between 4 and 6 micrometers, is formed by anisotropic etching using the silicon oxide film 211 as a mask, as shown in FIGS. 12B and 12E.

[0094] As shown in FIGS. 12C and 12F, after the damage due to the anisotropic etching is cured, a masking material 212 such as photoresist is deposited on the substrate 21 and defined by photolithography to unmask the longitudinal end of the trench 25. Then, the heavily doped region 26 is formed by off-axis ion implantation using the masking material 212 as a mask. After the masking material 212 is removed, the trench surface, which defines the trench 25, is isotropically etched by chemical dry etching using CF₄ and O₂ gasses by about 0.1 micrometers. Then, a sacrificial silicon oxide film, which has a thickness of about 100 nanometers, is formed by thermal oxidization in H₂O or O₂ atmosphere. The sacrificial oxide film is removed by wet etching using dilute fluoric acid. The silicon oxide film 211 may be simultaneously removed in this step.

[0095] Afterward, the first silicon oxide film 27 a, which has a thickness of about 100 nanometers, is formed by thermal oxidization in H₂O or O₂ atmosphere, as shown in FIGS. 13A and 13D. The first silicon oxide film 27 a is thicker at the end of the trench 25 than the rest of the first silicon oxide film 27 a, because the heavily doped region 26 is oxidized relatively faster, as shown in FIG. 13D. Then, the silicon nitride film 27 b, which has a thickness ranging between 10 and 30 nanometers, is deposited on the first silicon oxide film 27 a by LPCVD, as shown in FIGS. 13B and 13E. The silicon nitride film 27 b is anisotropically etched by chemical dry etching using etching gas containing CHF₃ and O₂ to leave the film 27 b only on sidewalls of the trench surface, as shown in FIGS. 13C and 13F.

[0096] Subsequently, the second silicon oxide film 27 c, the thickness of which is greater than 50 angstroms, is formed on the silicon nitride film 27 b by thermal oxidization in an H₂O or O₂ atmosphere at 950 degrees Celsius, as shown in FIGS. 14A and 14C. The entrance and bottom silicon oxide films 27 d, 27 e, which have a thickness of about 200 nanometers, are formed by this oxidization at the entrance and bottom of the trench 25, where the silicon nitride film 27 b is removed. A doped polycrystalline silicon film 213 is deposited by LPCVD to fill up the trench 25. Then, the doped polycrystalline silicon film 213 is defined to form the gate electrode 28 after the doped polycrystalline silicon film 213 is etched back to provide a predetermined thickness.

[0097] Although not illustrated, the interlayer insulating film 29 is formed by plasma CVD, and the contact hole extending through the film 29 is formed, and the source electrode 210 is formed by sputtering to complete the semiconductor device shown in FIGS. 11A and 11B.

[0098] Fourth Embodiment

[0099] As shown in FIG. 15B, the device according to the fourth embodiment has no heavily doped region 26 shown in FIG. 11B. Instead, the gate-insulating film 27 has a longitudinal end silicon oxide film 27 f (27 a, 27 c), which is thicker than the stack of films, at the longitudinal end of the trench 25. In this way, this device has the same effect as the device in the third embodiment.

[0100] The semiconductor device according to the fourth embodiment is manufactured with a process including steps shown in FIGS. 16A to 18D. The same steps as in FIGS. 12A, 12B, 12D, and 12E are carried out as shown in FIGS. 16A, 16B, 16D, and 16E. Afterward, as shown in FIGS. 16C and 16F, which show the same steps as in FIGS. 13A and 13D, the first silicon oxide film 27 a, which has a thickness of about 100 nanometers, is formed. Then, as shown in FIGS. 17A and 17D, which show the same steps as in FIGS. 13B and 13E, the silicon nitride film 27 b, which has a thickness ranging between 10 and 30 nanometers, is deposited on the first silicon oxide film 27 a. As shown in FIGS. 17B and 17E, which show the same steps as in FIGS. 13C and 13F, the silicon nitride film 27 b is etched to leave the silicon nitride film 27 b only on the sidewalls of the trench surface.

[0101] As shown in FIGS. 17C and 17F, which include the same photolithography step shown in FIGS. 12C and 12F, a masking material 220 such as photoresist is deposited on the substrate 21 and defined by photolithography to unmask the longitudinal end of the trench 25. Then, the unmasked silicon nitride film 27 b at the longitudinal end is removed by isotropic dry or wet etching, as shown in FIG. 17F. After the masking material 220 is removed, the surface that defines the trench 25 is oxidized, as shown in FIGS. 18A and 18C, which show the same steps shown in FIGS. 14A and 14C. By this oxidization, the second silicon oxide film 27 c is formed on the silicon nitride film 27 b. Simultaneously, the entrance silicon oxide film 27 d, the bottom silicon oxide film 27 e, and the longitudinal end silicon oxide film 27 f (27 a, 27 c), which have a thickness of about 200 nanometers, are formed respectively at the entrance, the bottom and the longitudinal end of the trench 25, where the silicon nitride film 27 b is removed. Then, the gate electrode 28 is formed, as shown in FIGS. 18B and 18D, which show the same steps as in FIGS. 14B and 14D.

[0102] Although not illustrated, after the gate electrode 28 is formed, the interlayer insulating film 29 is deposited. Then, a contact hole is formed in the interlayer insulating film 29 by photolithography and anisotropic etching, and the source electrode 210 is formed by sputtering to complete the semiconductor device shown in FIGS. 15A and 15B.

[0103] Fifth Embodiment

[0104] As shown in FIG. 19, the n⁺-type source region 24 includes a first region 24 a and a second region 24 b. The first region 24 a has a high enough impurity concentration to increase the oxidization speed of first region 24 a. The impurity concentration of the second region 24 b is lower than that of the first region 24 a so that the oxidization speed of the second region 24 b is not increased. The first region 24 a has a predetermined depth from the surface of the substrate. The depth of the second region 24 b is greater than that of the first region 24 a. The second region 24 b is located at the entrance of the trench 25 to separate the first region 24 a from the channel region, which is generated in the base region 23 in the vicinity of the trench 25. The thickness of the gate-insulating film 27 on the channel region is not affected by the thick silicon oxide film formed on the first region 24 a. Therefore, the threshold voltage of the semiconductor device of FIG. 19 is stable.

[0105] The first region 24 a and the second region 24 b are manufactured respectively by implanting n-type impurity ions into the base region 23 with a high concentration and a low concentration at the steps shown in FIGS. 12A and 12D or at the steps shown in FIGS. 16A and 16D, in which the n⁺-type source region 24 is formed.

[0106] Modifications

[0107] In the semiconductor device of FIGS. 11A and 11B, the heavily doped region 26 is formed after curing the damage due to the dry etching for forming the trench 25. However, the heavily doped region 26 may be formed at any step after forming the trench 25 and before forming the fist silicon oxide film 27 a. Moreover, the heavily doped region 26 may be formed before forming the trench 25 as long as the heavily doped region 26 has an appropriate impurity concentration.

[0108] In the semiconductor device of FIG. 19, the first region 24 a, the second region 24 b, and the trench 25 can be formed in this order. However, the order is flexible. For example, they may be formed in the following order: the second region 24 b, the first region 24 a, and the trench 25. Also, they may be formed in the following order: the trench 25, the first region 24 a, and the second region 24 b. In addition, the p-type base region 23 may be formed after forming the trench 25. For example, the trench 25, the p-type base region 23, the first region 24 a, and the second region 24 b may be formed in this order. In the latter example as well, it is possible to change the order of the first region 24 a and the second region 24 b.

[0109] The devices in FIGS. 11A, 15A, and 19 are n-channel transistors. However, the present invention may be used in a p-channel transistor.

[0110] While the present invention has been shown and described with reference to the foregoing preferred embodiments, it will be apparent to those skilled in the art that changes in form and detail may be made without departing from the scope of the invention as defined in the appended claims. 

1. A semiconductor device, comprising: a semiconductor substrate having a trench formed on a main surface thereof; and an insulating film disposed on an inner wall of the trench, the insulating film having a first portion composed of a first oxide film, a nitride film, and a second oxide film, and a second portion consisting of an oxide film, wherein: one of the first portion and the second portion of the insulating film is disposed on a side wall of the trench; and another one of the first portion and the second portion of the insulating film is disposed on at least one of an upper portion and a bottom portion of the trench.
 2. The semiconductor device according to claim 1, wherein: the first portion is disposed on the side wall of the trench; and the second portion is disposed on at least one of the upper portion and the lower portion of the trench and has a thickness larger than that of the first portion.
 3. The semiconductor device according to claim 2, wherein the second portion is disposed on both the upper portion and the bottom portion of the trench with the thickness larger than that of the first portion.
 4. The semiconductor device according to claim 2, wherein: the semiconductor substrate is composed of a first conductivity type first semiconductor layer, a second conductivity type second semiconductor layer, and a first conductivity type third semiconductor layer; the trench is formed from a main surface side of the semiconductor substrate, and penetrates the first semiconductor layer and the second semiconductor layer to reach the third semiconductor layer; the semiconductor substrate provides a transistor therein using the second semiconductor layer as a channel region, and the insulating film as a gate insulating film; and the nitride film of the first portion of the insulating film has an upper end at the main surface side that is located at a position closer to the main surface than a boundary between the first semiconductor layer and the second semiconductor layer.
 5. The semiconductor device according to claim 2, wherein: the oxide film of the second portion is formed by thermal oxidation that is performed for forming the second oxide film of the first portion after the first oxide film and the nitride film are formed on the side wall and on at least the one of the upper portion and the bottom portion, and after the nitride film on the one of the upper portion and the bottom portion is removed.
 6. The semiconductor device according to claim 1, wherein: the first portion of the insulating film is disposed on the bottom portion of the trench; and the second portion of the insulating film is disposed on the side wall of the trench.
 7. The semiconductor device according to claim 6, wherein: the semiconductor substrate is composed of a first conductivity type first semiconductor layer, a second conductivity type second semiconductor layer, and a first conductivity type third semiconductor layer; the trench is formed from a main surface side of the semiconductor substrate, and penetrates the first semiconductor layer and the second semiconductor layer to reach the third semiconductor layer; and the nitride film of the first portion of the insulating film has an upper end at the main surface side that is located at a position, a distance of which from the main surface is larger than that of a boundary between the second semiconductor layer and the third semiconductor layer.
 8. A trench-gate type transistor, comprising: a semiconductor substrate having a trench formed thereon; and a gate insulating film disposed on an inner wall of the trench, the gate insulating film including a first portion and a second portion respectively locally disposed on the inner wall of the trench and having different structures from each other, the first portion being composed of a first oxide film, a nitride film, and a second oxide film which are layered.
 9. The trench-gate type transistor according to claim 8, wherein a first region of the inner wall where the first portion of the gate insulating film is disposed includes a side wall portion of the trench in which a channel region is provided, and the first region excludes a bottom portion of the trench.
 10. The trench-gate type transistor according to claim 9, wherein the first region excludes an opening portion of the trench.
 11. The trench-gate type transistor according to claim 9, wherein a second region of the inner wall where the second portion of the gate insulating film is disposed includes at least one of an opening portion and the bottom portion of the trench.
 12. The trench-gate type transistor according to claim 11, wherein the second portion of the insulating film disposed on the one of the opening portion and the bottom portion of the trench has a thickness thicker than that of the first portion disposed on the side wall portion of the trench.
 13. The trench-gate type transistor according to claim 8, wherein: the first portion of the gate insulating film is disposed on a bottom portion of the trench; and the second portion consists of an oxide film and is disposed on a side wall portion of the trench.
 14. The trench-gate type transistor according to claim 8, wherein the second portion consists of an oxide film.
 15. A trench-gate type transistor, comprising: a semiconductor substrate having a trench formed thereon; and a gate insulating film disposed on an inner wall of the trench, the gate insulating film including a first portion and a second portion respectively located on first and second regions of the inner wall, the first portion being composed of a plurality of insulating films, the second portion being composed of only a single insulating film.
 16. The trench-gate type transistor according to claim 15, wherein: the first portion of the gate insulating film is disposed on a side wall portion of the trench; and the second portion of the gate insulating film is disposed at least on an opening portion and a bottom portion of the trench, and has a thickness larger than that of the first portion.
 17. The trench-gate type transistor according to claim 15, wherein the first portion of the gate insulating film is disposed on only a bottom portion of the trench.
 18. A method for manufacturing a semiconductor device, comprising: preparing a semiconductor substrate having a trench formed thereon; forming a first oxide film on an inner wall of the trench; forming a nitride film on the first oxide film; partially removing the nitride film so that the first oxide film is exposed at a first region of the inner wall and so that the nitride film remains on the first oxide film at a second region of the inner wall; forming a second oxide film, by thermal oxidation, on the first oxide film at the first region and on the nitride film at the second region.
 19. The method according to claim 18, wherein: the first region is at least one of an opening portion and a bottom portion of the inner wall; and the second region is a side wall portion of the inner wall.
 20. The method according to claim 19, wherein the second oxide film is formed so that a total thickness of the first oxide film and the second oxide film disposed on the first region is thicker than a total thickness of the first oxide film, the nitride film, and the second oxide film disposed on the second region.
 21. The method according to claim 18, wherein: the first region on which only the first oxide film and the second oxide film are disposed is a side wall portion of the inner wall; and the second region on which the first oxide film, the nitride film, and the second oxide film are disposed is a bottom portion of the inner wall.
 22. A method for manufacturing a trench-gate type transistor, comprising: preparing a semiconductor substrate composed of a first conductivity type first semiconductor layer, a second conductivity type second semiconductor layer, and a first conductivity type third semiconductor layer; forming a trench on the semiconductor substrate from a main surface of the semiconductor substrate at a side of the first semiconductor layer, the trench penetrating the first semiconductor layer and the second semiconductor layer to reach the third semiconductor layer; forming a gate insulating film on an inner wall of the trench by forming a first oxide film on the inner wall of the trench; forming a nitride film on the first oxide film; removing a part of the nitride film to expose a part of the first oxide film; and forming a second oxide film on the nitride film and on the part of the first oxide film; and forming a gate electrode in the trench.
 23. The method according to claim 22, wherein: when the gate insulating film is formed, the nitride film is partially removed to have an end portion at a side of the main surface of the semiconductor substrate, the end portion defining a distance from the main surface that is smaller than that of a boundary between the first semiconductor layer and the second semiconductor layer.
 24. The method according to claim 22, wherein: when the gate insulating film is formed, the nitride film is partially removed to remain only on a bottom portion of the trench and to have an end portion at a side of the main surface of the semiconductor substrate, the end portion defining a distance from the main surface that is larger than that of a boundary between the second semiconductor layer and the third semiconductor layer.
 25. The method according to claim 22, wherein: the second oxide film is formed on the nitride film at a first region of the inner wall and on the part of the first oxide film at a second region of the inner wall so that a total thickness of the gate insulating film at the second region is thicker than that at the first region.
 26. The method according to claim 25, wherein: the first region includes a side wall portion of the inner wall; and the second region includes at least one of an opening portion and a bottom portion of the trench.
 27. A method for manufacturing a trench-gate type transistor, comprising: a semiconductor substrate having a trench thereon; and forming a gate insulating film on an inner wall of the trench, wherein the formation of the gate insulating film is comprising: forming a first oxide film on the inner wall of the trench; locally disposing an oxidation preventive film on the first oxide film; and forming a second oxide film by thermal oxidation on the first oxide film with the oxidation preventive film locally interposed therebetween.
 28. The method according to claim 27, wherein the oxidation preventive film is a nitride film.
 29. The method according to claim 27, wherein: the oxidation preventive film is disposed on a side wall portion of the inner wall; the gate insulating film disposed on the side wall portion is composed of the first oxide film, the oxidation preventive film, and the second oxide film; and the gate insulating film disposed on at least one of an opening portion and a bottom portion of the trench is composed of only the first oxide film and the second oxide film.
 30. The method according to claim 27, wherein the oxidation preventive film is disposed only on a bottom portion of the trench.
 31. A semiconductor device comprising: a semiconductor substrate having a surface, in which a trench is formed; a heavily doped region, which is formed in a sidewall of a trench surface at a longitudinal end of the trench, and which is heavily doped with an impurity to increase oxidization speed of the heavily doped region; a stack of films, which is formed on the sidewall, and which includes: a first silicon oxide film, wherein the first silicon oxide film is thicker at the longitudinal end than that the rest of the first silicon oxide film; a silicon nitride film; and a second silicon oxide film, and a gate electrode formed on the stack of films.
 32. A semiconductor device comprising: a semiconductor substrate having a surface, in which a trench is formed; a stack of films, which is formed on a sidewall of a trench surface, and which includes: a first silicon oxide film; a silicon nitride film; and a second silicon oxide film, wherein the stack includes only the first and second silicon oxide films on the sidewall at a longitudinal end of the trench, and wherein the stack is thicker at a longitudinal end than the rest of the stack, and a gate electrode formed on the stack of films.
 33. The semiconductor device as in claim 31 or claim 32, wherein the device includes at least one of an entrance silicon oxide film, which is thicker than the stack, and which is located at an entrance of the trench, and a bottom entrance film, which is thicker than the stack, and which is located at a bottom of the trench.
 34. The semiconductor device as in claim 33, wherein: the substrate includes: a source region, which is a first conduction type; a base region, which is a second conduction type; and a drift region, which is the first conduction type, wherein the source region, the base region, and the drift region are located in this order from the surface; the trench extends perpendicularly from the surface to the drift region through the source region and the base region; and a channel region is generated in the base region adjacent to the trench.
 35. The semiconductor device as in claim 34, wherein: the source region includes: a first region, which has a predetermined impurity concentration and a predetermined depth; and a second region, which has an impurity concentration lower than the first region and a depth larger than the first region; and the second region separates the first region from the trench.
 36. A semiconductor device comprising: a substrate having a surface, wherein the substrate is a first or second conduction type; a drift layer, which is located on the substrate, and which is a first conduction type; a base region, which is located on the drift layer or embedded in a surface of the drift layer, and which is a second conduction type; a source region, which is embedded in a surface of the base layer, and which includes: a first region, which has a predetermined impurity concentration and a predetermined depth; and a second region, which has an impurity concentration lower than the first region and a depth larger than the first region, wherein: a trench extends perpendicularly from the surface of the substrate through the base region and reaches the drift region; and the second region separates the first region from the trench, a gate-insulating film formed on a sidewall of a trench surface; and a gate electrode, which is located on the gate-insulating film.
 37. A method for manufacturing a semiconductor device, in which a gate-insulating film is located on a sidewall of a trench surface, which defines a trench formed in a surface of a semiconductor substrate, and in which a gate electrode is located on the gate-insulating film, the method comprising steps of: forming a heavily doped region in the sidewall at a longitudinal end of the trench by doping an impurity with a concentration, at which oxidization speed of the doped region is increased; oxidizing the trench surface to form a first silicon oxide film, which is thicker at the heavily doped region than the rest of the first silicon oxide film; forming a silicon nitride film on the first silicon oxide film; oxidizing the trench surface to form a second silicon oxide film on the silicon nitride film.
 38. A method for manufacturing a semiconductor device, in which a gate-insulating film is located on a sidewall of a trench surface, which defines a trench formed in a surface of a semiconductor substrate, and in which a gate electrode is located on the gate-insulating film, the method comprising steps of: forming a first silicon oxide film on the trench surface; forming a silicon nitride film on the first silicon oxide film; removing the silicon nitride film at a longitudinal end of the trench; oxidizing the sidewall to form a second silicon oxide film on the silicon nitride film and to thicken the first silicon oxide film at the longitudinal end.
 39. The method as in claim 37 or claim 38, the method further comprising steps of: forming a drift region, which is a first conduction type; forming a base region, which is a second conduction type; forming a source region, which is a first conduction type, by forming a first region, which has a predetermined impurity concentration and a predetermined depth, and a second region, which has an impurity concentration lower than the first region and a depth larger than the first region; and forming the trench, which extends perpendicularly from the surface of the substrate to the drift region through the source region and the base region, such that the second region separates the first region from the trench.
 40. The method as in claim 37 or claim 38, the method further comprising a step of removing the silicon nitride film from at least one of an entrance and a bottom of the trench to form a silicon oxide film thicker than the stack.
 41. A method for manufacturing a semiconductor device, the method comprising steps of: providing a substrate, which is a first or second conduction type; forming a drift layer, which is a first conduction type, on the substrate; forming a base region, which is a second conduction type, on the drift layer or in a surface of the drift layer; forming a source region in a surface of the base region by making a first region, which has a predetermined impurity concentration and a predetermined depth, and a second region, which has an impurity concentration lower than the first region and a depth larger than the first region; forming a trench, which extends through the source region and the base region and reaches the drift layer, such that the second region separates the first region from the trench; forming a gate-insulating film on a trench surface; and forming a gate electrode on the gate-insulating film.
 42. A semiconductor device comprising: a semiconductor substrate having a surface in which a trench is formed, wherein the trench has a wall; a heavily doped region, which is formed in the wall at an end of the trench, wherein the heavily doped region is doped with an impurity to increase the oxidization speed of the heavily doped region; a stack of films formed on the wall, wherein the stack includes: a first silicon oxide film, wherein the thickness of the first silicon oxide film is greater at the end of the trench than elsewhere; a silicon nitride film; and a second silicon oxide film, and a gate electrode formed on the films.
 43. The semiconductor device of claim 42, wherein the device includes at least one of an entrance silicon oxide film, the thickness of which is thicker than that of the stack, located at an entrance of the trench, and a bottom film, the thickness of which is greater than that of the stack, located at a bottom of the trench.
 44. The semiconductor device of claim 43, wherein the substrate includes: a source region of a first conduction type; a base region of a second conduction type; and a drift region of first conduction type, wherein the source region, the base region, and the drift region are located in this order from the surface and the trench extends perpendicularly from the surface to the drift region through the source region and the base region; and a channel region in the base region adjacent to the trench.
 45. The semiconductor device of claim 44, wherein the source region includes: a first region, which has a predetermined impurity concentration and a predetermined depth; and a second region, which has an impurity concentration that is lower than the first region, wherein the depth of the second region is greater than that of the first region, and the second region is located between the first region and the trench.
 46. A semiconductor device comprising: a semiconductor substrate having a surface in which a trench is formed, wherein the trench has a wall and an end; a stack of insulating films formed on the wall, wherein the stack includes: a first silicon oxide film; a silicon nitride film, which is located on the first silicon oxide film; and a second silicon oxide film, which is located on the silicon nitride film; and an end insulating film formed at the end of the trench, wherein the end insulating film includes only the first and second silicon oxide films, and the thickness of the end insulating film is greater than that of the stack, and a gate electrode formed on the stack and on the end insulating film.
 47. The semiconductor device of claim 46, wherein the device includes at least one of an entrance silicon oxide film, the thickness of which is thicker than that of the stack, located at an entrance of the trench, and a bottom entrance film, the thickness of which is greater than that of the stack, located at a bottom of the trench.
 48. The semiconductor device of claim 47, wherein the substrate includes: a source region of a first conduction type; a base region of a second conduction type; and a drift region of first conduction type, wherein the source region, the base region, and the drift region are located in this order from the surface and the trench extends perpendicularly from the surface to the drift region through the source region and the base region; and a channel region in the base region adjacent to the trench.
 49. The semiconductor device of claim 48, wherein the source region includes: a first region, which has a predetermined impurity concentration and a predetermined depth; and a second region, which has an impurity concentration that is lower than the first region, wherein the depth of the second region is greater than that of the first region, and the second region is located between the first region and the trench.
 50. A semiconductor device comprising: a substrate of a first or second conduction type, wherein the substrate has a surface; a drift layer of the first conduction type, wherein the drift layer is located on the surface of the substrate; a base layer of a second conduction type, wherein the base layer is located on the drift layer or embedded in a surface of the drift layer; a source region, which is embedded in a surface of the base layer, wherein the source region includes: a first region, which has a predetermined impurity concentration and a predetermined depth; and a second region, which has an impurity concentration that is lower than the first region, wherein the depth of the second region is greater than that of the first region; a trench that extends perpendicularly from the surface of the substrate through the base region and reaches the drift region, wherein the second region is located between the first region and the trench; a gate-insulating film formed on a wall of the trench; and a gate electrode located on the gate-insulating film.
 51. A method for manufacturing a semiconductor device, in which a gate-insulating film is located on a wall of a trench formed in a semiconductor substrate, wherein a gate electrode is located on the gate-insulating film, the method comprising: forming a heavily doped region in the wall at an end of the trench by doping an impurity with a concentration such that the oxidization speed of the doped region is increased; oxidizing the trench surface to form a first silicon oxide film, which is thicker at the heavily doped region than elsewhere; forming a silicon nitride film on the first silicon oxide film; and oxidizing the trench surface to form a second silicon oxide film on the silicon nitride film.
 52. The method of claim 51 further comprising: forming a drift region of a first conduction type; forming a base region of a second conduction type; forming a source region of the first conduction type by forming a first region, which has a predetermined impurity concentration and a predetermined depth, and a second region, which has a lower impurity concentration and a greater depth than the first region; and forming the trench to extend perpendicularly from the surface of the substrate to the drift region through the source region and the base region, such that the second region is located between the first region and the trench.
 53. The method of claim 51 further comprising removing the silicon nitride film from at least one of an entrance and a bottom of the trench to form a silicon oxide film thicker than the stack.
 54. A method for manufacturing a semiconductor device, in which a gate-insulating film is located on a wall of a trench formed in a semiconductor substrate, wherein a gate electrode is located on the gate-insulating film, the method comprising: forming a first silicon oxide film on the wall; forming a silicon nitride film on the first silicon oxide film; removing the silicon nitride film at an end of the trench; oxidizing the wall to form a second silicon oxide film on the silicon nitride film and to thicken the first silicon oxide film at the end of the trench.
 55. The method of claim 54 further comprising: forming a drift region of a first conduction type; forming a base region of a second conduction type; forming a source region of the first conduction type by forming a first region, which has a predetermined impurity concentration and a predetermined depth, and a second region, which has a lower impurity concentration and a greater depth than the first region; and forming the trench to extend perpendicularly from the surface of the substrate to the drift region through the source region and the base region, such that the second region is located between the first region and the trench.
 56. The method of claim 54 further comprising removing the silicon nitride film from at least one of an entrance and a bottom of the trench to form a silicon oxide film thicker than the stack.
 57. A method for manufacturing a semiconductor device, the method comprising: providing a substrate of a first or second conduction type; forming a drift layer of the first conduction type on the substrate; forming a base region of the second conduction type on the drift layer; forming a source region in a surface of the base region by making a first region, which has a predetermined impurity concentration and a predetermined depth, and a second region, which has a lower impurity concentration and a greater depth than the first region; forming a trench, which extends through the source region and the base region and reaches the drift layer, such that the second region is located between the first region and the trench; forming a gate-insulating film in the trench; and forming a gate electrode on the gate-insulating film. 